String#rindex should handle matchdata strings

A string extracted from a matchdata object shares the same
byte array as the string it was matched against. When doing a
rindex between a string from a matchdata and the string the matchdata
was matched against the same byte array is passed into ByteList.memcmp
which will always return 0 as ByteList.memcmp just checks for equality
of the byte arrays.

I fix this by passing bytes() instead getUnsafeBytes() into the
ByteList.memcmp method so that the byte arrays are different.

Fixes #1688
Merge pull request #1714 from atambo/rindex_with_matchdata

String#rindex should handle matchdata strings